Recently, in semiconductor materials, demands for thinner and lighter one are becoming stronger. In silicon wafers for semiconductor use, it is required to reduce the thickness to 100 μm or even less, but such a thin wafer is very brittle and easily cracked. Therefore, in wafer processing, a method of supporting the wafer on a temporary fixing pressure-sensitive adhesive sheet, and, after a necessary processing, releasing and recovering the wafer has been adopted.
Such temporary fixing pressure-sensitive adhesive sheet generally includes an energy ray-curable pressure-sensitive adhesive layer, and is employed in a method of adhering it to a wafer, then, after a processing such as grinding or dicing is applied to the temporarily fixed wafer, curing the pressure-sensitive adhesive layer by an irradiation with an energy ray and peeling off the pressure-sensitive adhesive layer with a lowered pressure-sensitive adhesive power from the wafer. However, the pressure sensitive adhesive sheet, of which the pressure-sensitive adhesive power is lowered by the irradiation with the energy ray, still sticks to the wafer surface by the atmospheric pressure. Therefore, in order to peel off the pressure-sensitive adhesive sheet from the wafer, there is required an operation, for example, of pulling off the pressure-sensitive adhesive sheet, but such operation is liable to cause troubles such as an edge chipping or a breakage of the wafer, for example, by a stress at the operation. Furthermore, when the thickness of the wafer is reduced after polishing (for example, when reduced to about 25 μm), the end edges of the pressure-sensitive adhesive sheet attached to the wafer may protrude further to the outside compared to the wafer edges. Thus, there is a risk that these protruding parts may adhere to the surface of a working table or to the part of dicing tape that is provided on the polished surface of the wafer, or the like, thus causing difficulties in peeling.
Japanese Patent No. 3073239 discloses a pressure-sensitive adhesive sheet including an energy ray-curable pressure-sensitive adhesive layer and a thermally shrinkable film in a layered structure. According to this pressure-sensitive adhesive sheet, as the heat-shrinkable sheet contracts at the irradiation with energy ray, an elongation or creases generated in the sheet, for example at irradiation with ultraviolet light, can be prevented. However, the peelability of the pressure-sensitive adhesive sheet from the wafer is still insufficient.
JP-A-2000-129227 discloses a pressure-sensitive adhesive sheet for protecting a semiconductor wafer, formed by a shrinkable film, a stiff film and an energy ray-curable pressure-sensitive adhesive layer. According to this pressure-sensitive adhesive sheet, the adhesive power of the pressure-sensitive adhesive layer is lowered by an irradiation with an energy ray and the shrinkable film is made to contract by required means, thereby causing a deformation in the pressure-sensitive adhesive sheet and reducing the contact area between the wafer and the pressure-sensitive adhesive layer, so that the pressure-sensitive adhesive sheet can be easily peeled off from the wafer. However, as the inventors of the invention selected arbitrary materials and examined similar ones, the inventors found that since the contraction of a shrinkable film occurring in multiple directions causes irregular deformation of the pressure-sensitive adhesive sheet after heating such that the pressure-sensitive adhesive sheet is folded and overlaps on the wafer surface, difficulties in peeling or adherend failure may occur. Generally, even a uniaxially shrinkable film available commercially causes a subsidiary contraction (one or plural contractions with relative weak shrinking power) in an axial direction different from the main shrinking axial direction due to a residual stress at the manufacture or due to a stress or a thermal strain applied to the pressure-sensitive adhesive sheet in the course of manufacture, and the contraction occurs as a combination of these.
When the area of the adherend is small, the deformation of the pressure-sensitive adhesive sheet caused by such subsidiary shrinkage is small, so that the detriments at the peeling tend to occur less. However, with an increase in the dimension of the adherend, the subsidiary contraction also increases and may eventually hinder the contraction in the direction of the principal shrinking axis. Particularly, in a pressure-sensitive adhesive sheet having a dimension widely used for a wafer, such inhibition of contraction in the direction of principal shrinking as is liable to occur, thus leading to troubles such as an incomplete peeling in which a part of the pressure-sensitive adhesive sheet remains adhered to the wafer after the peeling, a breakage in the adherend by an uneven stress at the contraction, and a contamination on the wafer by the pressure-sensitive adhesive cured and dropping from the pressure-sensitive adhesive sheet. It was also found that in the bilayer constitution of the substrate and the pressure-sensitive adhesive, the warpage generated after polishing the wafer was so large that conveyability and the like were very poor.
Patent Reference 1: Japanese Patent No. 3073239
Patent Reference 2: JP-A-2000-129227